In the field of Non-Destructive Testing surface analysis, known techniques include visual inspection, liquid penetrant, and magnetic particle inspection. In addition, microscopy techniques such as confocal, interferometric, scanning electron microscopy (SEM) and mechanical surface profile instruments such as one- and two-dimensioned styles based instruments are known techniques to determine a surface a profile. Within the class of visual inspection techniques, surface replication may be used to aid in the remote inspection of materials. Known surface replication techniques involve the transfer of a surface profile to a media such as a replica tape for indirect examination using microscopy. Known replica tape uses a cellulose acetate film to transfer surface profiling for imaging microstructures with standard microscopy techniques. However, this technique is not used for surface profiling but rather typically as a means to ascertain image crack formation or creep damage of components.
Other known methods of surface transfer to a replicating media include replication putty, shape memory polymer, as well as casting techniques using two-part silicon rubbers and methacrylate. The resulting replica is analyzed using a number of techniques including interferometric, scanning electron microscopy (SEM), and confocal microscopy. One or two-dimensional mechanical surface profile instruments can also be used on the replica media, such as drag stylus or needle type surface profile gages.
It is known to use replica tape to characterize a surface. Method C of ASTM D4417 describes the process for measuring the peak to valley distance of a surface, such as abrasive blasted surfaces on metal bridges and ships, for example. The character of the blasted surface is predictive of paint adherence. If the peak to valley distance is too small, the surface lacks sufficient “tooth” to anchor the paint. If the peak to valley distance is too large, the high peaks may protrude through the paint to become foci for corrosion.
PRESS-O-FILM®, which is an example of a commercially available replica tape, includes a non-compressible 50 μm (2 mil) polyester backing and a compressible layer of foam. The backing is attached to an adhesive backed paper carrier. The carrier has an approximately 8 mm diameter opening exposing the polyester and foam below which is embossed or compressed against the surface to be characterized causing the foam to conform to the surface. The thickness of the replica tape is then measured with a spring loaded micrometer, and the thickness of the polyester backing is subtracted to indicate the peak to valley distance of the original surface. However, the micrometer is not capable of providing other measurements, such as the peaks per unit area, the mean peak area, or developed surface area. These additional statistics may be useful in further predicting paint adherence and longevity.
Replica tape provides a permanent record of the surface. Some micrometers can also store the thickness reading of the replica tape to provide a permanent record of the peak to valley measurement. Such micrometers, however, do not provide additional measures of the surface.
Replica tape is particularly useful for measurements on curved surfaces that are difficult to measure directly with stylus instruments or interferometric laser scanning or optical focal distance measuring devices.
The use of imaging techniques in conjunction with replica media is known. Both reflective and backlit techniques have been employed to analyze surfaces. Commercially available metalized replica tape is useful for scanning electron microscope characterization of the surface. Metalized replica tape is also useful for interferometric characterization of the surface, for example, when the original surface is insufficiently reflective. However, these techniques are neither low cost nor portable.
Most materials exhibit absorbance to radiation at particular wavelengths. This is known as the Beer-Lambert law. It is known that a mapping of thickness to absorbance can yield a 3D image. However, known surface analysis techniques do not involve deriving a calibrated thickness mapping of such an image. For example, U.S. 2003/0222215 discloses a technique of determining the thickness of a thin layer coated on a surface of a sample by taking a picture of the section to be examined to obtain a digital image of that section. An intensity profile in the thickness direction of the thin layer is extracted from the digital image and is then analyzed based on characteristics of the intensity profile that are independent from properties of the sample, such as sample thickness, a radius of curvature of the thin layer in a thickness direction of the thin layer, and a tilt angle introduced during the preparation of the sample. However, U.S. 2003/0222215 involves the use of expensive radiation equipment such as an x-ray machine in order to emit electron radiation to ascertain an intensity profile in the thickness direction of the thin layer at an atomic level. Furthermore, U.S. 2003/0222215 does not provide for any mechanism for performing in situ calibration adjustments of the profile based on the corresponding thickness measurements.
Exemplary embodiments of the present disclosure provide an apparatus and method to derive a thickness profile of a replication media by using an independent thickness measurement means to calibrate a characteristic intensity profile. The apparatus is specifically constructed to derive a calibrated thickness profile of a replication media such as a replica tape, for example.